USB is an external bus standard that specifies the electrical connections and data transfer operations needed to enable electronic devices to interface and communicate with each other. USB is a serial interface that is often used in place of RS232 serial interfaces and parallel interfaces to connect peripheral devices (e.g., mice, keyboards, printers, etc.) to computers (e.g., desktop and laptop computers). Most desktop and laptop computers on the market today are equipped with multiple USB connectors, each of which is designed to mate with a respective USB plug. A typical USB connector is configured with electrical contacts that are designed to couple to electrical contacts external to the USB connector in order to perform data transfer and power supply functions. Some of the electrical contacts of the USB connector are used to couple electrical contacts of a USB plug to the electrical circuitry of the USB connector, whereas some of the electrical contacts of the USB connector are used to couple the electrical circuitry of the USB connector to conductive traces formed on a motherboard of a computer. Electrical traces on the motherboard route electrical signals between the electrical circuitry of the USB connector and electrical circuitry mounted on the motherboard, such as, for example, a USB controller that is connected to a main processor of the computer.
In recent years, owing to the increasing amount of data traffic between computers and their peripheral devices, USB connectors have been equipped to support increasing speeds: e.g., increases from the 10 megabits per second (Mbps) speed provided by the USB1 standard to the 480 Mbps and 5 gigabits per second (Gbps) speeds provided by the USB2 and USB3 standards, respectively. There continues to be a demand for computer-to-peripheral communications that operate at even higher speeds. For example, most high-resolution, real-time video will require data rates above 10 Gbps. At speeds above 10 Gbps, the conventional copper wire connections used for USB devices will become difficult to implement and will have limited reach. Consequently, using an optical connection with backward compatibility to earlier versions of USB connections becomes highly desirable.
In any USB connection implemented in a computer, the data traffic is managed by a USB controller IC mounted on the motherboard of the computer. The controller IC is electrically coupled via traces on the motherboard with the main processor of the computer on one side, and with a physical layer device that conditions the signal for proper transmission on the other side. The physical layer device then connects via traces on the motherboard to the USB connector. In the conventional USB connections, a copper wire based cable is plugged into the USB connector to enable electrical signals to be routed between the computer and its peripheral devices. To introduce an optical connection to the USB connector, an electrical-to-optical/optical-to-electrical conversion module (OE module), which performs the functions of optical-to-electrical conversion and electrical-to-optical conversion, takes the place of the aforementioned physical layer device. In general, two types of arrangements are used for implementing the optical connection. In one of the two types of arrangements, the OE module is mounted on the motherboard through an electrical socket soldered on the motherboard or by direct soldering of the OE module electrical contacts to contact pads on the motherboard. An optical fiber jumper cable is used to provide an optical connection between the OE module and the optical connector inserted in a USB connector receptacle housing that is mounted on the motherboard of the computer. The external USB cable is modified to contain two optical fibers terminated in an optical connector that is inserted inside of the USB plug that terminates the USB cable.
The optical jumper cable has a first connector on a first end thereof, which mates with the USB connector, and a second connector on a second end thereof, which mates with an optical connector that is an integral part of the OE module. The jumper cable typically includes one transmit optical fiber and one receive optical fiber per USB connector. Similarly, the OE module typically includes one laser diode to transmit optical signals and one photodiode to receive optical signals per USB connector. However, if support for multiple USB connections is desired, an OE module can contain multiple laser diodes and an equal number of multiple photodiodes and connect to a jumper cable having the corresponding number of jumper fibers. Electrical contacts of the OE module are electrically coupled via conductive traces on the motherboard to a controller device (e.g., a USB controller IC) mounted on the motherboard. The controller device is electrically coupled via traces on the motherboard to the main processor of the computer.
The OE module includes electrical driver circuitry that receives electrical signals carried on traces of the motherboard from the router and converts them into electrical drive signals that are used to drive the laser diode of the OE module. The corresponding optical signals that are produced by the laser diode are then optically coupled by an optics system of the OE module into the ends of the transmit fiber secured to the second connector of the jumper cable. When optical signals are received in the OE module over the receive fiber of the jumper cable, the optics system of the OE module optically couples the received optical signals onto the photodiode of the OE module. The photodiode produces corresponding electrical signals that are routed from the OE module over traces of the motherboard to the controller device. The controller device then routes the electrical signals over traces of the motherboard to the main processor or another processor of the computer.
In the other of the two types of arrangements, the OE module, which is equipped with an integrated passive optical connector, is contained within the USB connector. This passive optical connector directly couples optical signal to and from the passive optical connector situated inside of the USB plug attached at the end of the aforementioned external optical USB cable. Therefore, no jumper fibers are used in this arrangement. A flex circuit is connected on one end thereof to the OE module and on the opposite end thereof to an electrical connector mounted on the computer motherboard. This electrical connector interfaces the flex circuit with the motherboard. Electrical signals are routed on traces of the motherboard from the main processor to the controller device and from the controller device to the interface of the flex circuit and the motherboard. The electrical signals are then routed over traces of the flex circuit to the OE module contained within the USB connector.
In the OE module, the electrical signals are used to drive the laser diode to produce optical signals. The optical signals produced by the laser diode of the OE module are optically coupled via the optics system of the OE module and the integrated passive optical connector into the passive optical connector contained inside of the USB plug at the end of the transmit fiber contained within the modified optical USB cable. At the opposite end of the transmit fiber, the optical signals are received in the passive optical connector contained within the USB plug, which, in turn, couples the optical signals to the passive optical connector of the OE module contained in the USB receptacle housing; the photodiode inside of the OE module on the opposite end receives the optical signals via the optics system, and the receiver circuitry of the OE module produces the corresponding electrical signals. The electrical signals are then routed over traces of the flex circuit onto traces of the motherboard via the electrical interface between the flex circuit and the motherboard. The electrical signals are then routed over traces of the motherboard to the controller device and then from the controller device to the main processor.
The main disadvantages of the two types of arrangements of the optical connections described above result from the many interfaces that are needed to transfer signals between the USB connector and the controller device mounted on the motherboard. In the arrangement that uses the optical fiber jumper cable, reflection losses, also commonly referred to as Fresnel losses, and additional optical losses caused by misalignment of the optical elements, are possible at each of the optical interfaces. In order to keep the overall USB connector cost down, the ends of the transmit and receive fibers are typically cleaved, but left unpolished, which can result in unpredictable losses occurring at locations where optical signals are coupled into or out of the ends of the fibers. Also, in some cases, a refractive index-matching epoxy is used to attach the fiber ends to an optical element, such as a lens of the optics system. Bubbles can occur at the tip of the optical fiber in the epoxy, which can potentially result in losses occurring at each interface where the optical signal encounters a portion of the bubble (i.e., when the signal encounters the outer surface of the bubble and again when the signal encounters the inner surface of the bubble). Therefore, the use of the optical fiber jumper cable creates many opportunities for optical losses to occur, which can degrade signal quality. While the other type arrangement of the optical connections essentially eliminates the external optical jumper cable, it requires an additional electrical connection for interfacing the flex circuit with the motherboard. This additional electrical connection creates the potential for signal losses and connectivity problems to occur, which can degrade electrical signal quality. In addition, the electrical connection has a cost associated with it that increases the overall cost associated with the USB connector. Also, the manufacturing process involving the flex circuit generally suffers from lower throughput, and possibly more variability than would result using a hard printed circuit board (PCB).
Accordingly, a need exists for a USB connector that has optical capabilities for handling optical signals and which does not require the use of an optical jumper cable or a flex circuit, thereby eliminating the aforementioned problems associated with those types of optical USB arrangements.